I put my other flow-jet pump in and removed original.
I also have after market facet with pull out tubing.
I removed the facet completely and turned on the water.
It fluctuates. Sometimes the pump revs fast and sometimes very slow.
The toilet flush is GOOD the bathroom facet is GOOD.
The shower is now doing the same thing more often.
I Can hear the pump revving up and down.
I even filled the water tank all the way up.
Hope this is enough information to get closer to the culprit at hand. Oh, and FYI all works GOOD using city pressure.